In recent years, a variety of power storage devices, for example, nonaqueous secondary batteries such as lithium-ion secondary batteries (LIBs), lithium-ion capacitors (LICs), and air cells have been actively developed. In particular, demand for lithium-ion secondary batteries with high output and high energy density has rapidly grown with the development of the semiconductor industry, for electronic devices, for example, portable information terminals such as cell phones, smartphones, and laptop computers, portable music players, and digital cameras; medical equipment; next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs); and the like. The lithium-ion secondary batteries are essential as rechargeable energy supply sources for today's information society.
A negative electrode for power storage devices such as lithium-ion secondary batteries and the lithium-ion capacitors is a structure body including at least a current collector (hereinafter referred to as a negative electrode current collector) and an active material layer (hereinafter referred to as a negative electrode active material layer) provided over a surface of the negative electrode current collector. The negative electrode active material layer contains an active material (hereinafter referred to as a negative electrode active material) which can receive and release lithium ions serving as carrier ions, such as carbon or silicon.
At present, a negative electrode of a lithium-ion secondary battery which contains a graphite-based carbon material is generally formed by mixing graphite as a negative electrode active material, acetylene black (AB) as a conductive additive, PVDF, which is a resin as a binder, to form a slurry, applying the slurry over a current collector, and drying the slurry, for example.
Such a negative electrode of a lithium-ion secondary battery and a lithium-ion capacitor has an extremely low electrode potential and a high reducing ability. For this reason, an electrolytic solution containing an organic solvent is subjected to reductive decomposition. The range of potentials in which the electrolysis of an electrolytic solution does not occur is referred to as a potential window. The potential of the negative electrode needs to be within the potential window of an electrolytic solution. Most of the potentials of negative electrodes of lithium-ion secondary batteries and lithium-ion capacitors are, however, out of the potential windows of all electrolytic solutions; thus, electrolytic solutions are subjected to reductive decomposition and passivating films (also referred to as solid electrolyte films) are formed as decomposition products on the surfaces of the negative electrodes. The passivating films inhibit further reductive decomposition, which enables insertion of lithium ions into the negative electrodes with the use of low electrode potentials out of the potential windows of the electrolytic solutions (for example, see Non-Patent Document 1).    [Non-Patent Document 1] Zempachi Ogumi, “Lithium Secondary Battery”, Ohmsha, Ltd., the first impression of the first edition published on Mar. 20, 2008, pp. 116-118